The present invention relates generally to a shuttle printer which carries out printing on a printing sheet while shuttling a hammer bank back and forth in a direction to traverse the printing paper, wherein the hammer bank carries a plurality of dot print hammers juxtaposed along a print line and is driven by a shuttle motor. More particularly, the invention relates to an apparatus for controlling a reversing duration of the hammer bank at constant, during which time the printing paper is fed a predetermined amount.
In such a shuttle printer, typically one or two dot lines are printed simultaneously during one scan of the hammer bank defined by the movement thereof from the left end to the right end or vice versa, and a printing paper is subsequently fed one or two dot lines in the reversing duration of the hammer bank defined by a period of time at which the hammer bank turns around at the left or right end. This one scan printing and the subsequent paper feeding are repeatedly carried out a predetermined number of times to thus print one print line, and thereafter the printing paper is fed four or six dot lines within the hammer bank reversing duration to thus provide the corresponding amount of interline space between the printed line and the following print line. In the shuttle printer in which printing is carried out on one or two dot line basis, the reversing duration of the hammer bank is sufficiently long in completing the paper feeding for providing the interline space.
There has been known a shuttle printer whose print hammer assembly has an arrangement such that six dot lines can be printed at a time. U.S. Pat. No. 4,889,052 discloses such a shuttle printer in which six dot lines are printed simultaneously during one scan of the hammer bank and the printing paper is fed twelve dot lines after completion of printing for one print line to provide corresponding amount of interline space. In this shuttle printer, the paper feeding for the interline space must also be completed within the reversing duration of the hammer bank. To this end, it is necessary that the reversing duration be precisely controlled because a larger amount of paper feeding must be performed within a limited period of time. If the hammer bank reversing duration is varied in the direction to be shortened, the interline space would be constricted or the paper settling time could not be preserved, with the result that the print quality is degraded. If the reversing duration of the hammer bank is set longer to obviate the above-mentioned drawbacks, a high speed printing cannot be achieved notwithstanding the fact that it is the initial contemplation to increase the printing speed by the provision of the print hammer assembly which can print six dot lines at a time with one scan of the hammer bank.
Nonetheless, it has been difficult to maintain the hammer bank reversing duration at constant. The reversing durations at which the hammer bank turns around at the right and left ends are different from each other due to the variation in precision of a shuttle mechanism. Further, the reversing duration changes depending upon the change of the load of the shuttle motor due to wear of the mechanism caused by aging.
Further, in a shuttle printer having dual printing mode, i.e., a high-speed and low-speed printing modes, there is a demand for changing the reversing duration depending upon the mode selected. In the high-speed printing mode, alpha/numerals are typically printed line by line at an interval of 30 dot lines in which 18 dot lines are allocated to the height of alpha/numerals and the rest of 12 dot lines to the interline space. Assuming that 6 dot lines are printed simultaneously during one scan of the hammer bank, printing for one print line is carried out in such a way that 6 dot lines are printed in the initial rightward scan of the hammer bank, the printing paper is fed 6 dot lines during a period of time when the hammer bank turns around at the left end, another six 6 dot lines are printed in the subsequent leftward scan, the printing paper is again fed 6 dot lines for a period of time when the hammer bank turns around at the right end, then still another 6 dot lines are printed in the rightward scan of the hammer bank, and thereafter the printing paper is fed 18 dot lines to have the interline space. In this manner, in the high-speed printing mode, the 18 dot line feeding is done within the reversing duration of the hammer bank. In the low-speed printing mode, on the other hand, the hammer bank reversing duration is set to a different value in conformity with the printing mode. In any event, it has been necessary to control the hammer bank reversing duration at constant depending upon the mode selected.